This invention relates to a system of communication employed during the drilling of boreholes in the earth for purposes such as oil or gas exploration and production, the preparation of subterranean services ducts, and in other civil engineering applications.
Taking the drilling of oil and gas wells as an example, it is highly desirable both for economic and for engineering reasons, to obtain information about the progress of the borehole and the strata which the drilling bit is penetrating from instruments positioned near the drilling bit, and to transmit such information back to the surface of the earth without interruption to the drilling of the borehole. The generic name associated with such techniques is xe2x80x9cMeasurement-while-Drillingxe2x80x9d (MWD). Substantial developments have taken place in MWD technology during the last twenty-five years.
One of the principal problems in MWD technology is that of reliably telemetering data from the bottom of a borehole, which may lie several thousand meters below the earth""s surface. There are several established methods for overcoming this problem, one of which is to transmit the data, suitably encoded, as a series of pressure pulses in the drilling fluid; this method is known as xe2x80x9cmud pulse telemetryxe2x80x9d.
A typical arrangement of a known mud pulse MWD system is shown schematically in FIG. 1. A drilling rig (50) supports a drillstring (51) in the borehole (52). Drilling fluid, which has several important functions in the drilling operation, is drawn from a tank (53) and pumped, by pump (54) down the center of the drillstring (55) returning by way of the annular space (56) between the drillstring and the borehole (52). The MWD equipment (58) that is installed near the drill bit (59) includes a means for generating pressure pulses in the drilling fluid. The pressure pulses travel up the center of the drillstring and are received at the earth""s surface by a pressure transducer (57). Processing equipment (60) decodes the pulses and recovers the data that was transmitted from downhole.
In one means of generating pressure pulses at a downhole location, the fluid flowpath through the drillstring is transiently restricted by the operation of a valve. This creates a pulse, the leading edge of which is a rise in pressure; hence the method is colloquially, although rather loosely, known as xe2x80x9cpositive mud pulse telemetryxe2x80x9d. In contradistinction the term xe2x80x9cnegative mud pulse telemetryxe2x80x9d is used to describe those systems in which a valve transiently opens a passage to the lower pressure environment outside the drill string, thus generating a pulse having a falling leading edge.
Devices for generating pulses for positive mud pulse telemetry have been described in, for example, U.S. Pat. Nos. 3,958,217, 4,905,778, 4,914,637 and 5,040,155.
The present invention is related generally to the type of mud pulse generator described in U.S. Pat. No. 3,958,217. It is a disadvantage of this type of pulse generator that the magnitude of the transient pressure change which occurs downhole is highly dependent on the flowrate of the drilling fluid.
The pressure drop when fluid flows through a restriction varies approximately as the square of the flow rate. Typically, the ratio of maximum to minimum flow rates in an oilwell drilling situation is around three, so a pulse generator set up to give an acceptable pulse height of around 7 bar at minimum flow of a particular drilling mud formulation would give 63 bar at maximum flow. In practice, drilling mud is formulated with a wide range of densities and viscosities, so the potential variation in pulse height across the flow range is considerably greater.
Although in any given drilling situation a certain minimum pulse amplitude is needed so that the pulse will be detectable at the earth""s surface, it is unsatisfactory for the pulse to be made too large: the imposition of a succession of severe flow restrictions can stress, damage or erode the drilling equipment and starve the drilling bit of fluid. Furthermore, when mud pressure pulses are too large, significant pulse reflections occur at discontinuities in the process pipework. In particular a pulse can return to the lower end of the drillstring, be reflected, return to surface and be detected, incorrectly, as a data pulse.
In order to keep pulse heights within acceptable limits, the pulse generator has to be physically adjusted to suit a particular combination of flow rate and mud type. This typically involves replacing parts of the downhole system, and is time consuming and expensive. There are cases too, in which for unexpected reasons, the planned flowrates for a particular well section have to be changed while the equipment is downhole. It is therefore very desirable to provide a single system which will operate satisfactorily over a wide range of drilling fluid flowrates.
The invention seeks to obtain this advantage by providing a means of automatic pulse height regulation in the fluid used in a drilling installation.
According to the invention there is provided a pressure pulse generator for use in transmitting pressure signals to surface in a fluid-based drilling system, said generator being arranged in use in the path of a pressurised fluid to operate a drilling assembly and being capable of being actuated to generate pressure signals in such fluid for transmission to surface pressure monitoring equipment, in which the pulse generator comprises:
a housing positionable in the path of the supply of pressurised fluid, said housing having an inlet arrangement for admitting a portion of the fluid to the interior of the housing, and an outlet arrangement for discharging fluid from the interior of the housing;
a control element slidably mounted in the housing for movement between an open position and a closed position with respect to said inlet arrangement, said control element being operative to generate a pressure pulse in the supply of pressure fluid when the control element takes-up the closed position;
a control passage for receiving a portion of the supply of pressure fluid and extending through the control element, and having an inlet at one end to receive pressure fluid and a discharge outlet at an opposite end;
a valve element arranged to be exposed to the pressure of the fluid in the control passage;
an actuator coupled with the valve element and operative to move the valve element between a closed position in which it prevents discharge of pressure fluid from the control passage, and an open position in which it allows the pressure fluid to flow through the control passage;
a control face on the control element which is exposed to the pressure of the fluid in the control passage and which is operative to move the control element towards the closed position with respect to the inlet arrangement as the pressure in the control passage increases upon movement of the valve element to the closed position by the actuator; and,
a resiliently yieldable arrangement acting between the actuator and the valve element in order to define a yieldable limit to the pressure of the fluid in the control passage and thereby control the pressure pulse generated by the movement of the control element to the closed position.